# The Cost of Driving Additional Weight

April 2017DISCLAIMER: I’m pretty sure my calculations & logic are correct but I’m not confident so take everything with a pinch of salt.

With that out of the way, here’s some quick context. Two week ago, I ran out of water so I’ve just been drinking fucking tea and coffee for hydration. Anyways, on a recent Costco run, I finally picked up a pack of water bottles, which I didn’t immediately bring up to my room. That was a couple of days ago so I’ve just been trucking this pack of water bottles around town with me. So just earlier, my friend was ribbing me about it, saying I was wasting money not bringing the water up. Instead of bringing it up, I wrote this post.

A quick google search shows that every additional 100 lbs should roughly reduces gas mileage by 1-2%. That’s pretty low but the author doesn’t explain the number and I’m thinking “how hard could this be” so I attempted to calculate it myself.

From this wiki on energy density, Gasoline is 34.2 MJ per litre. Engine efficiency is roughly 20%, so we have 6.84 MJ of usable energy per litre. I can’t find information on the weight of my 2015 Hyundai Elantra so I’ll use 4,009 pounds which this article cited as the average weight of American cars in 2010. I couldn’t find the 0-60 time of my car, so I used the time for the 2016 Hyundai Elantra Value Edition and calculated acceleration from that.

Using the above, we can calculate a rough estimate of what we want, here are the calculations:

I figured that if I just compared the difference in work for two different weights, I wouldn’t have to calculate all this stuff like air drag and friction. I know hardly any physics so my process to find the cost per km is pretty trivial. Essentially I found the difference in work attributable to the change in weight which was then translated into cost. The result was a cost per km coming out to 1.27 cents which is 1.2% of gas price. That’s a bit above the value of 1-2% per 100 lbs that was mentioned earlier, but it’s still pretty close (considering my calculations oversimplified the situation)